I am trying to write a feature to avoid the hyphenation of certain digraphs in Basque language (e.g. tx, tz, dd, tt...). Hyphenation plug-ins don't work very well for Basque and disconnecting those digraphs is considered unacceptable in a formal text. I thought it could be solved through localized substitutions (locl) of these combinations by their respective ligatures. Apparently I was wrong. Ligatures seem to substitute the digraphs at a presentation level, but they are still coded as two letters (which make sense on the other hand) and they remain subject to hyphenation when the edge of the column is close enough.
I also considered using wordjoiner (U2060) or zerowidthjoiner (200D) to keep the two letters together but a) It would involve a third glyph and I don't know if such substitution is posible (I mean, substituting two glyphs by three), and b) if the former is possible, would it remain the same word, a different word or two distinct words?